Orthogonal Pin Construct Versus Parallel Uniplanar Pin Constructs for Pelvic External Fixation: A Biomechanical Assessment of Stiffness and Strength

Objective: The purpose of this study is to compare the structural stiffness of an orthogonal pelvic external fixator pin construct with 2 different parallel external fixator pin constructs in a simulated bone model. Hypothesis: An orthogonal pelvic external fixator pin construct would be significantly more stiff than a parallel pin construct when loaded in-plane under similar conditions. Design: Thirty synthetic pelvic bone models were configured with orthogonal pins (group 1), parallel iliac crest pins (group 2), or parallel supra-acetabular pins (group 3). Specimens were loaded either in-plane (flexion/extension moment) or out-of-plane (internal/external rotation moment) to assess construct stiffness. Setting: Orthopaedic industry mechanical testing laboratory (Stryker Orthopedics, Mahwah, NJ). Intervention: Single load cycle to failure with load application modified to assess stiffness both in-plane and out-of-plane with the pin constructs. Main Outcome Measure: Pelvic external fixation pin construct stiffness. Results: Stiffness for in-plane loading was 150.2 +/- 51.2 N/mm for the orthogonal pin construct, 105.0 +/- 46.9 N/mm for the iliac crest pin construct, and 104.7 +/- 20.7 N/mm for the supra-acetabular pin construct. Pairwise comparisons demonstrated that the difference was significant (P < 0.05) between groups 1 and 2 and groups 1 and 3 but not between groups 2 and 3. Stiffness for out-of-plane loading was 49.6 +/- 3.4 N/mm for the orthogonal pin construct, 53.9 +/- 3.5 N/mm for the iliac crest pin construct, and 100.6 +/- 4.3 N/mm for the supra-acetabular pin construct, with significant differences (P < 0.05) between groups 1 and 3 and groups 2 and 3 but not between groups 1 and 2. Conclusion: An orthogonal pelvic external fixator pin construct produced a significantly stiffer construct for in-plane loading (flexion/extension moment) compared with either parallel pin construct; however, a parallel supra-acetabular pin construct was stiffer for out-of-plane loading.